In general, capacitive MEMS pressure sensors include a fixed electrode that forms one plate of a parallel plate capacitor and a movable electrode that forms the other plate of the parallel plate capacitor. The fixed electrode is typically provided in a substrate, such as a silicon wafer. The movable electrode is incorporated into a deformable membrane that is suspended over the fixed electrode on the surface of the substrate. The deformable membrane is configured to deflect toward the substrate under an applied pressure which alters the gap between the fixed electrode and the movable electrode, resulting in a change in the capacitance between the two electrodes. By monitoring the change in capacitance between the fixed electrode and the movable electrode, a magnitude of a pressure applied to the deformable membrane can be determined.
The electrodes may be formed in a variety of different ways, such as by the deposition of a conductive film, electrical isolation of a conductive layer, adding a spacer layer between two conductive layers, and implant doping of the silicon substrate. In some capacitive MEMS pressure sensors, the deformable membrane and movable electrode are formed by an epitaxially deposited polysilicon (“epi-poly”) cap layer. During fabrication of the sensor, the cap layer is deposited onto a sacrificial oxide layer formed on the substrate in the area of the fixed electrode. The sacrificial layer is then removed between epi-poly cap layer and the substrate to release the membrane and form a cavity between the movable electrode in the membrane and the fixed electrode in the substrate that defines the capacitive gap.
The geometry and dimensions of the epi-poly cap layer and the capacitive gap can be tailored to a certain degree to provide pressure sensors with a desired level of accuracy, sensitivity, and/or linearity. Higher pressure sensitivity typically requires more flexible membranes while higher accuracy and linearity require less flexible membranes. Membrane flexibility is determined in part by the size (e.g., lateral extent) and thickness of the membranes. Membranes with greater flexibility can typically be achieved by increasing the size or lateral extent of the membrane and/or by reducing the thickness of the membrane. Conversely, membranes with higher accuracy and linearity can be achieved by decreasing the lateral extent of the membrane and/or by increasing the thickness of the membrane.
The ability to achieve a desired level of accuracy, sensitivity, and/or linearity for a sensor depends at least in part on the precision and uniformity of the layers used to define the structures of the device, particularly the sacrificial oxide layer that defines the capacitive gap between the epi-poly cap layer and the substrate. A precise and uniform sacrificial layer promotes a precise and uniform capacitive gap in the sensor which in turn improves performance of the sensor by more closely matching design targets and by decreasing variations across the device.
The precision and uniformity of the sacrificial layer is dictated by the processes used to fabricate the sensor. Previously known fabrication methods typically used either a low pressure chemical vapor deposition (LPCVD) process or a thermal oxidation process to form the silicon dioxide sacrificial layer. A LPCVD process allows thicker depositions (e.g., >2 um), but the precision and uniformity of the depositions are limited. Thermal oxidation enables excellent precision and uniformity (e.g., several angstroms over an entire wafer), but the process is much slower. Therefore, thicker depositions require exposing the substrate to elevated temperatures for prolonged periods which may not be allowed by the thermal budget of the fabrication process.